1. Field of the Invention
The present invention relates generally to an acoustic or ultrasonic microscope, and in more particular concerns an ultrasonic microscope which has a specimen table held in a contactless manner.
2. Description of the Prior Art
In recent years, generation and detection of acoustic waves at ultra high frequency of up to 1 GHz have succeeded with an acoustic wavelength of about 1 .mu.m being made available in water. In reality, an ultrasonic imaging equipment of a high resolution has been realized for practical applications, in which a focused ultrasonic beam is generated with the aid of a concave lens system, allowing a resolution as high as 1 .mu.m to be attained.
In an application of such ultrasonic imaging equipment, a specimen is inserted in an ultrasonic beam path, wherein an ultrasonic wave reflected by the specimen is detected and processed for studying elastic properties and the like of an extremely small area of the specimen. In another application, a specimen is scanned in two dimensions, wherein the resulting signals are displayed as brightness signals on a cathode-ray tube or CRT display with a desired magnification to facilitate observation of an extremely small given region of the specimen.
A typical one of the hitherto known acoustic microscope imaging equipments has been disclosed in U.S. Pat. No. 4,028,933. To have a better understanding of the invention, description will first be made in some detail on the prior art structure of such equipment.
Referring to FIG. 1 of the accompanying drawings which schematically illustrate a general structure of a probe or transducer system constituting a main part of a hitherto known ultrasonic microscope, an ultrasonic wave propagating medium 20 which may be a cylindrical crystal body of, for example, sapphire or silica glass has one end face 21 having a plane polished to an optical quality and the other end face which is formed with a concave semispherical hole 30. An RF pulse ultrasonic wave which is a plane wave is radiated into the crystal 20 by an RF pulse signal which is applied to a piezo-electric film 10 deposited on the end face 21. The plane ultrasonic wave is focused onto a specimen 50 located on a predetermined focal point through a positive acoustic lens formed at the interface between the semi-spherical hole 30 and a medium 40 which is usually water.
The ultrasonic wave reflected and scattered by the specimen 50 is collected and converted into a plane wave by means of the same lens. The plane wave is propagated through the interior of the crystal 20, and is finally converted into an RF electric signal by the piezo-electric film 10. The RF electric signal is detected by a diode circuit to be converted into a video signal, which is then utilized as the input signal of the CRT display mentioned above.
There are shown in FIG. 2 at (a) signal waveforms of a video frequency range produced in response to application of the RF pulse signal with a predetermined repetition rate t.sub.R in the hitherto known structure shown in FIG. 1. In FIG. 2, the abscissas represents a time axis while the intensity or magnitude of the signal is taken along the ordinate. A letter A designates the applied RF pulse, a letter B designates a reflected signal from the interface lens and a letter C designates a reflected signal from the specimen.
In order to discriminate the desired reflected signal C from the reflected signal B, the hitherto known imaging equipment adopts such an arrangement in which the duration t.sub.d (FIG. 2 (b)) of the impressed pulse is selected as short as possible so as to prevent the signals C and B from overlapping each other, whereby only the signal C is extracted through an appropriate timing gate operation, as illustrated in FIG. 2 at (c).
By the way, the resolution of such equipment includes an axial or depth resolution .DELTA..rho. in the direction of propagation of the ultrasonic wave and a bearing resolution .DELTA..gamma. in a plane extending in the direction perpendicular to the propagating direction of the ultrasonic wave. Both of these resolutions are determined by the wavelength .lambda. of the ultrasonic wave and the F number representative of the brightness of the lens as used, and are given by: EQU .DELTA..gamma.=.lambda..multidot.F (1) EQU .DELTA..rho.=2.lambda..multidot.F.sup.2 ( 2)
Since the F number of the lens which can be realized is on the order of 0.7, the resolutions .DELTA..gamma. and .DELTA..rho. will be, respectively, about 1 .mu.m and 1.5 .mu.m in the water (1,500 m/s) when the ultrasonic wave used is at 1 GHz.
For IC's or LSI's which are one of the most important objects to be examined by the ultrasonic microscrope, more improved axial resolution is required, because layer patterns in the thicknesswise or depthwise direction of the specimen is often finer than a planar pattern, as is well known. In actuality, a typical IC has a multilayered structure composed of layers of 1 .mu.m to 3 .mu.m thick. With the axial resolution of 2 .mu.m in the water as described above, it is utterly impossible to observe these layers independent of one another in a non-destructive manner with the position of a focal point set inwardly of the surface of the IC. This can be explained by the fact that, since the acoustic velosity is higher in a metal such as silicon and aluminum constituting the IC than in the water, the axial resolution is only from 4 to 10 .mu.m even when the ultrasonic wave at 1 GHz is used.